Armor protection is usually provided by steel or other homogenous material plates which retard and hopefully prevent the penetration of incoming missiles, primarily by material shear strength and strain energy to failure. Even if penetration is prevented, plasmas and shock waves resulting from the missile can cause break up, usually tensile failure, of the inside wall of the plate. This results in the armor plate itself creating missiles, known as spall, which can injure personnel and damage equipment. The armor may be in the form of several plates in series which can move and chop the projectile or the plasma jet created by the projectile, thereby diverting the projectile. Such plates slide as they are penetrated and are referred to as Chobham armor. Spall nevertheless can result upon impact of the Chobham armor. The homogeneous nature of the armor, whether as individual slabs or slidable plates, provides a good conductor of shock waves which is the major contributor for the spall. Reactive armor has been suggested which explodes on impact. The explosion opposes the incoming shock wave and penetrator, causing their diversion. So called explosive or reactive armor even if modularized can explode in unison causing even more severe damage to the vehicle on which the reactive armor is mounted than the incoming threat. Also, the replacement of the reactive armor modules is difficult.
In order to provide complete ballistic protection against high kinetic energy threats, even in the case of lightweight ballistics (excluding hyper velocity threats or shaped charges) requires a heavy armor, when steel or steel plates are used. Even armor using ceramic or plastics, such as Kevlar(1) (aramid fibers) does not provide complete lightweight ballistic protection. Lightweight ballistics which must be completely stopped and can be stopped in accordance with the invention are typified by the following data:
Type I—Material-Tungsten; velocity 4920 ft/sec; shape-sphere; diameter 0.63 inch; weight 40 grams; and kinetic energy (KE)=33117 ft-lb.
Type II—material-tungsten; velocity 2600 ft/sec; shape cylindrical (L/D=4) (where L is the length of the cylinder and D is its diameter); diameter 0.33 inches; weight 25.4 grams; and KE 5873 ft-lb. Calculations and tests indicate that to be effective in providing complete lightweight ballistic protection to these threats the foregoing steel plate-type armor would weigh at least 140 pounds per square foot of frontal area. It will be apparent that such protection would add an impractical weight burden in the case of aircraft, and would add such a load that it would be practical only for powerful vehicles such as tanks and half-tracks or on large ships. Aircraft so protected could not become airborne. For example, helicopters cannot be so overloaded and be operable. Revetments and walls would also be so heavy as to preclude their practical portability. Shelters so protected could not be portable but would necessarily be permanent installations which would not meet tactical objectives. Accordingly, the problem remains to provide effective ballistic protection without the severe weight penalty imposed by conventional armor. It is also desirable to provide complete protection without the dangers incident to the use of reactive, explosive armor. (1) Dupont Trade Name
It has been calculated that an armor structure, embodying the invention, providing complete ballistic protection equivalent to steel armor having an estimated weight of 140 pounds per square frontal foot would weigh only 49 pounds per square frontal foot. The weight would be less when threats having less energy than those listed above are to be protected against. For less kinetic energy bullets or ballistic fragments, an area density of about 9.77 pounds per square frontal foot can be achieved with armor structures embodying the invention. This is based upon employing only one 3″×3″×¾″ thick tile element such as shown in FIG. 1 oriented obliquely (e.g. 45 degrees) to the incoming threat. With modules all layed-up at 0° facing toward the threat field the weight would be only 8.63 pounds per square foot of frontal area because fewer tiles are needed to span the area to be protected. Also, for many threats even thinner tiles (less than ¾″) will be needed which will further reduce the necessary weight. Accordingly, ballistic protection of the type provided by bullet proof vests can be provided utilizing the invention. Also, semi-permanent or quickly and easily positionable walls of revetments and shelters (for example, for the protection of aircraft) may be provided in accordance with the invention.
An additional problem is presented by the environment. Temperature extremes, moisture and wind, in addition to many chemicals such as solvents, acid, alkalize, fuels, hydraulic fluids and salt spray must be tolerated. Armor structures provided in accordance with the invention, unlike steel and other chemically sensitive materials, are adapted to handle and be operative for long periods of time under severe environmental conditions such as noted above.